1. Field of the Invention
The present invention relates to a SQUID (Superconducting QUantum Interference Device).
2. Related Background Art
A SQUID is constructed by forming at least one Josephson junction in a superconducting loop and used for an element such as a magnetometer capable of accurately measuring a magnetic field or a gradiometer.
Josephson junctions of some types are used in superconducting loops, and one of them is a step-edge Josephson junction. In the step-edge Josephson junction, a step having a predetermined shape is formed on a substrate on which a superconducting loop is to be formed by a superconducting thin film pattern. A superconducting thin film pattern is formed on the substrate such that the step crosses a predetermined portion of the superconducting loop by a superconducting thin film. At this time, a Josephson junction is obtained using weak link formed at the superconducting thin film pattern portion on the step.
To form the above-described step-edge Josephson junction using a thin film pattern formed from an oxide superconducting thin film, an SrTiO3 substrate is generally used as a substrate because a satisfactory oxide superconducting thin film can be formed (Jpn. J. Appl. Phys., Vol. 32 (1993), pp. 662-664). However, an SrTiO3 substrate is expensive, and a large SrTiO3 substrate is unavailable. Since the number of SQUIDs which can be manufactured from a single wafer substrate decreases, the manufacturing efficiency is low.
For a SQUID magnetometer, to increase the magnetic field detection sensitivity, the area of a SQUID may be required to be large by, e.g., making the SQUID size large to increase the flux capture area. However, the above-described SQUID can hardly cope with an increase in area.
The present invention has been made to solve the above-described problems, and has as its object to provide a SQUID which has a step-edge Josephson junction and is capable of attaining cost reduction and large area of the element.
In order to achieve the above object, according to the present invention, there is provided a SQUID comprising a sapphire substrate, and a superconducting thin film pattern formed from an oxide superconducting thin film formed on the sapphire substrate via a CeO2 buffer layer and including a SQUID pattern, wherein a Josephson junction included in the SQUID pattern comprises a step-edge Josephson junction formed on a step on the sapphire substrate.
The sapphire substrate used in the above SQUID is relatively inexpensive, and the substrate size can be made large. However, a technique of manufacturing a SQUID by forming a step-edge Josephson junction in forming a superconducting thin film pattern on a sapphire substrate has not been established.
In accordance with the results of examinations and experiments conducted by the present inventor, when a superconducting thin film pattern formed from an oxide superconducting thin film is formed on a sapphire substrate via a CeO2 buffer layer, and a step is formed on the sapphire substrate such that it crosses a predetermined portion of the thin film pattern, a step-edge Josephson junction for causing the element to perform the SQUID operation can be obtained. The present inventor completed the present invention on the basis of this finding. Hence, the SQUID having a step-edge Josephson junction can be made inexpensive. In addition, since a large sapphire substrate can be used, the area of the SQUID can be increased, and the manufacturing efficiency can be improved.
The present invention will be more fully understood from the detailed description given hereinafter and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description,